The present invention relates generally to formation of colloidal metal nanoparticles and films of colloidal metal nanoparticles. In particular, the present invention relates to colloidal metal nanoparticles prepared by hydroxylamine seeding and monolayers of such colloidal metal nanoparticles.
The use of nanosized colloidal Au nanoparticles has expanded greatly in recent years. Whereas ten to fifteen years ago, the predominant use of colloidal Au was in biological transmission electron microscopy, a wide variety of recent papers now describe interesting physical properties and possible applications that extend far beyond imaging. For example, colorimetric DNA sensors based on colloidal Au have been developed. Moreover, organized two-dimensional (2-D) and three-dimensional (3-D) arrays of colloidal Au nanoparticles are now occupying the attention of several groups. Unless one studies metal nanoparticles one at a time, as has recently been described, understanding the behavior of solutions and/or surfaces containing Au nanoparticles is predicated upon having a single size (and shape) of particle because, in the nanometer regime, almost every relevant physical property of colloidal Au is size dependent. Photolithography and, more recently, nanosphere lithography have been used to prepare surfaces with highly regular metal features. However, for materials synthesis and for manipulations of these particles in solution, nanoparticles prepared from metal ions in solution are the preferred starting materials.
Many different preparations have been reported for the synthesis of colloidal Au, including even some that begin with bulk metal. However, most such preparations begin with Au3+ and, through use of different reductants, generate particles with a range of particle sizes. For example, reducing agents such as NaBH4 or white phosphorous produce small Au particles (diameter  less than 10 nm), while reductants such as ascorbic acid yield colloidal Au nanoparticles with diameters larger than 10 nm. The most widely-studied reductant is sodium citrate; by varying the citrate:Au ratio, it is possible to prepare colloidal particles with diameters (d) ranging from 10-150 nm. Unfortunately, for d greater than 30 nm, the monodispersity becomes poor and the ellipticity (G)xe2x80x94the ratio of the major to minor axisxe2x80x94significantly exceeds unity, the value for a sphere. As a result, these particles are of limited value for nanometer-scale architecture. Indeed, of the many published routes to colloidal Au, none produce large ( greater than 50 nm) particles with good monodispersity. Their utility is further restricted by the low particle concentrations generated by these methods. For example, EP 426300B1 describes 2.6-nm diameter colloidal Au nanoparticles (xe2x80x9cseedsxe2x80x9d) prepared by BH4xe2x88x92 reduction grown to larger sizes by addition of a boiling solution of HAuCl4 and citrate. This approach produces large colloidal Au particles more predictably and reproducibly than citrate reduction, but because the seeds are highly polydisperse (40% standard deviation), the monodispersity is comparable.
Formation of conductive metal films by faradaic and non-faradaic deposition onto immobilized metal nanoparticles is a widely-used process in industry, and of significant recent interest. The focus of this work has typically been on production of thin films exhibiting high conductivity and good adhesion, with special attention given to micron-to-submicron control of film thickness and ease of fabrication. Though largely successful, two aspects of film growth by electroless metal deposition have received little attention. The first is that the number of metal nanoparticles used to nucleate film growth is usually not a controllable parameter. As a result, detailed mechanistic information about particle coalescence is lacking. The second is that the analogous processes in solutionxe2x80x94that is, enlargement of suspended metal nanoparticlesxe2x80x94have not been studied. As a result, information about the size and shape of growing particles is unavailable.
Accordingly, there remains a need for metal nanoparticles with narrow size distributions, and methods for making them. There also remains a need for methods for controlled growth of nanoparticles in Au colloid monolayers, multilayers, and in solution, along with the control of desired physical characteristics of such monolayers, multilayers, and solutions.
Ensembles of nanoparticles display unique optical and electrical properties that are distinct from their respective bulk properties or simply the average measurement of individual particles. To a large extent, however, bulk material properties (i.e., catalytic, optical, electrical, biocompatibility) are determined by nanoscale features. The ability to tune particle, size, shape, chemical composition, array geometry and linking chemistries provides a flexible platform to manipulate material properties through rational design of the principal components (i.e., metal or semiconductor nanoparticles).
Materials composed from 2-D and 3-D ensembles of nanoparticles are becoming increasingly important in analytical and materials chemistries; indeed, practical applications in nanoelectronic and optoelectronic devices, chemical sensors, and catalysis seem imminent. For example, arrays of crystalline modified polystyrene spheres and suspended ensembles of ligand-coated metal nanoparticles are finding use as vapor phase molecular recognition sensors. Self-organized 2-D nanoparticle superlattices of latex spheres, CdS, CdSe, Au, and Ag structures have been constructed and analyzed. Organized 3-D arrays of nanoparticles with inter- and intra-layer particle registry have been assembled from polystyrene, Ag, CdS, and inorganic oxide nanoparticles. However, with the exception of the inorganic oxides reported by Stein and coworkers, (see Holland, et al., Chem. Mater. 1999, 11:795-805; and Holland, et al., Science 1998, 281:802-804), no assemblies extend more than a few layers above the substrate and none offer any control over film thickness.
Interest in 2-D metal nanoparticle arrays stems from several unique characteristics: (i) Concentrated solutions of monodispersed Au nanoparticles from 2-100 nm in diameter are easily synthesized. Metal nanoparticles readily adsorb onto appropriately derivatized surfaces. Typically, organosilanes, hyperbranched polymers, or alkylthiols are used to generate arrays with random packing but with a reproducible overall coverage and with a reasonable distribution of interparticle spacing. (ii) Optical properties are a function of particle spacing, size, and composition, easily tailored attributes. (iii) Particles have a high surface area, useful for applications in catalysis, electrochemistry, biomolecule conjugation, and surface-sensitive spectroscopies. In contrast to sol-gel or polymer encapsulation, where the majority of the particle is coupled to the matrix and inaccessible to solution or gas phase chemistry, only a small fraction of an individual particle is in contact with the surface. (iv) Fabrication of patterned collections of nanoparticles has potentially important implications in nanoelectronic device fabrication and biosensing. In this regard, Natan and coworkers have previously characterized Au colloid monolayer synthesis, rate of assembly, thermodynamics, and morphology; extension of assemblies into 3-D architectures may lead to new properties and broadened applications. See Grabar, et al., Anal. Chem. 1995, 67:735-743; Grabar, et al., Langmuir 1996, 12:2353-2361; Grabar, et al., J. Am. Chem. Soc. 1996, 118:1148-1153; Keating, et al., J. Chem. Educ. 1999, 76:949-955; and Grabar, et al., Anal. Chem. 1997, 69:471-477.
Accordingly, there remains a need for the development of a general methodology for assembling bulk metal-like films directly from solution in a stepwise fashion. The present invention overcomes the limitations of the prior art and further illustrates possible applications in electrochemistry, biosensors, conductive coatings, surface patterning, and optical devices.
The present invention provides a method for preparing colloidal metal nanoparticles, in which a solution of reductant is added to a boiling solution of a source of metal ion coincidentally with seed colloids. In a preferred embodiment, the colloidal metal nanoparticles are colloidal gold nanoparticles, the source of gold ions is HAuCl4, and the reductant is sodium citrate. The present invention is also directed toward a method for preparing colloidal metal nanoparticles, in which seed colloids are added to a solution of reductant mixed with a solution containing a source of metal ions. In a preferred embodiment, the colloidal metal nanoparticles are colloidal gold nanoparticles, the source of gold ions is HAuCl4, and the reductant is NH2OH. In a further embodiment, the seed colloids are the result of the application of the method, and the source of metal ions and the reductant are added to the seed colloids in an iterative fashion. The present invention is also directed to the colloidal metal nanoparticles that are the product of the application of the above methods. In one embodiment, colloidal metal nanoparticles have a major axis standard deviation of about 14% or less, a minor axis standard deviation of about 14% or less, and an ellipticity  less than 1.5. In another embodiment, colloidal metal nanoparticles have a major axis standard deviation of about 29% or less, a minor axis standard deviation of about 20% or less, and an ellipticity  greater than 1.5.
The present invention is also directed toward a method for enlarging immobilized gold nanoparticles using a similar method of immersing an Au colloid monolayer in a solution of reductant, and adding a solution having a source of Au3+. The present invention is also directed towards methods for the preparation of thin gold films whose nanostructure can be varied based on the amount of coverage of Au colloid in the monolayer. In the case of a low-coverage Au colloid monolayer, a gold film with features up to about 50 nm tall and about 75 nm wide results. In the case of a high-coverage Au colloid monolayer, a gold film with features about 25 nm tall or less results.
The present invention is also directed toward the gold films prepared by these methods, including a thin Au film having substantially similar conductivity to pure Au, a thin Au film useful as an a cyclic voltammetry electrode, a thin Au film whose nanostructure is indistinguishable from those of granular metal films prepared by evaporation, and having a mean roughness of about 3 nm rms, a thin Au film with increased SERS intensity, and a thin Au film for which a reflectivity vs. excitation angle plot exhibits a pronounced minimum.